The present invention relates to a method for leaching zinc concentrate in atmospheric conditions, so that the obtained end product is a zinc sulfate solution, which is further conducted into electrolysis, and iron precipitate, which advantageously is jarosite precipitate.
In the prior art, there is known a method described in the EP patent publication 451,456, according to which zinc concentrate is roasted by conventional methods and conducted into neutral leaching. The ferrite that is left undissolved is conducted into strong-acid leaching, and into the strong-acid leaching step there is also fed bulk concentrate, which contains, in addition to zinc sulfide, also lead and precious metals. In the neutral leaching step, the zinc oxide of the calcine dissolves into zinc sulfate and is conducted to electrolysis after the purification steps. In the strong-acid leaching step, there is created a precious metals and lead containing precipitate, which is conducted either to pyrometallurgical treatment or to flotation in order to recover the precious metals and lead. The solution created in the strong-acid leaching step also contains the dissolved iron in ferrous form, wherefore the solution is conducted into iron removal, which according to the invention is carried out in an autoclave and in oxidizing conditions, and the iron is precipitated as hematite. The solution from which the iron is separated is then conducted to the neutral leaching step. This method is particularly applicable when the zinc concentrate contains precious metals and lead.
In the prior art there is also known the method described in the U.S. Pat. No. 4,676,828, where part of the concentrate is roasted and part subjected to direct leaching. The roasted part of the concentrate is dissolved in a two-step neutral leaching, and the undissolved ferrites are conducted to a multi-step direct leaching of the zinc concentrate. At least one step of the direct leaching takes place at heightened pressure and temperature, which increases the costs of the process. The valuable metals of the concentrate are separated from the direct leaching precipitate, and the dissolved iron is precipitated from the zinc sulfate solution by means of a calcine addition. Iron is precipitated as goethite.
From the U.S. Pat. No. 4,274,931 there is known the recovery of zinc from zinc sulfide concentrate; zinc concentrate is leached in conditions where the temperature is within the region of 70-119xc2x0 C., the quantity of iron and other impurities is 5-50 g/l and the quantity of sulfuric acid 20 g/l at maximum. Leaching takes place in two steps, in the first of which the concentrate is leached with solution obtained from the second leaching step, so that there occurs a simultaneous leaching of the zinc contained in the concentrate and a partial precipitation of the iron contained in the solution, and in the second step there occurs a simultaneous leaching of the iron precipitate and of the concentrate that was left undissolved into he first step. From the first leaching step there is obtained a sulfate solution containing zinc and iron. This solution is conducted into the zinc calcine leaching circuit, where iron is precipitated by neutralizing it with the zinc calcine, whereafter the rest of the impurities are removed and the solution conducted into electrolysis. The leaching precipitate of the first step, which still contains some undissolved concentrate and precipitated iron, is in the second step subjected to leaching with the return acid from the electrolysis. Moreover, oxygen is fed to this step. From the precipitate left from the second step leaching, there is separated elemental sulfur and undissolved sulfide by means of flotation.
The U.S. Pat. No. 3,959,437 describes an extremely economical method for recovering zinc form roasted zinc concentrate. The zinc oxide of the calcine is leached in a neutral leaching step, and the ferrites that are left undissolved in this step are leached in a separate conversion step, where there is simultaneously carried out both the leaching of ferrite and the precipitation of iron as jarosite.
As was already mentioned in the references above, the dissolution of zinc sulfide concentrate in a sulfuric acid solution occurs through intermediation of trivalent iron according to the following reaction equations:
xe2x80x83ZnS+Fe2(SO4)3xe2x86x92ZnSO4+FeSO4+S0xe2x80x83xe2x80x83(1)
The created bivalent iron is regenerated by means of oxygen:
2FeSO4+H2SO4+{fraction (1/2)}O2xe2x86x92Fe2(SO4)3+H2Oxe2x80x83xe2x80x83(2)
FeS, which is in the zinc sulfide lattice, reacts in similar fashion as zinc sulfide:
FeS+Fe2(SO4)3xe2x86x923 FeSO4+S0xe2x80x83xe2x80x83(3)
The ferrous iron obtained into the solution must be precipitated, and this can be carried out either as goethite, jarosite or hematite. If iron should be very far precipitated as goethite, as is described in the U.S. Pat. No. 4,274,931, the pH must be raised high with respect to the iron precipitation conditions, and in such conditions zinc is dissolved very slowly. This means that iron must be precipitated in a separate step by using for instance zinc calcine as precipitation agent, as is done in point 8 of said U.S. patent.
Hematite can be precipitated in a higher acid content than goethite, and so that zinc sulfide dissolves effectively and serves as the neutralizing agent, as is described in the EP patent 451,456, but then autoclave conditions must be applied.
The precipitation of iron as jarosite may take place in atmospheric conditions with an acid content that is so high that zinc sulfide is dissolved, if the rest of the conditions are arranged in a favorable manner. As is well known, iron is precipitated as jarosite in atmospheric conditions (low temperatures) very slowly, and the precipitation is surface-activated. In order to achieve a sufficient precipitation speed, it is advantageous, particularly with higher acid contents, that an adequately high jarosite concentration exists in the precipitation situation. This is achieved for example by recirculating jarosite, as is described for instance in the Canadian patent 1,094,819.
The present invention relates to a method for leaching zinc concentrate in atmospheric conditions in the presence of trivalent iron. It is essential that the zinc concentrate is fed into conditions where in addition to trivalent iron, there are also present jarosite nuclei, where the sulfuric acid content of the leaching step is kept in the region 10-40 g/l and the temperature in the region 80xc2x0 C.xe2x80x94the solution boiling point, and where into the leaching step there is fed oxygen, so that the zinc concentrate is dissolved and the iron is precipitated as jarosite. The essential novel features of the invention are apparent from the appended patent claims.
We have now proved that it is advantageous to feed the zinc concentrate to a conversion process, where the zinc contained in the concentrate is leached and the iron is precipitated simultaneously. In that case the concentrate is fed directly to the conversion step. From the point of view of zinc recovery, this method is an huge improvement, because now the leaching can be combined with the conversion step, and a complicated process is not needed at all. The method is as simple as the conversion method, and the recovery as good. This method also enables an improved recovery of zinc from ferrite, because in the novel method the acid level of the final part of the conversion step can be raised, because the iron precipitation need not be brought as far as in a conventional conversion process, because the rest of the iron can be precipitated in connection with the leaching of the zinc concentrate. The elemental sulfur created in the leaching of the zinc concentrate is either conducted to the jarosite residue or recovered as a separate process step.